This invention relates to semiconductor lasers and, more particularly, to configurations thereof that provide wavelength tunability. More specifically, the present invention is a diode laser which can be operated at selectably variable frequencies covering a wide wavelength range comprising, active section semiconductor means for creating a light beam by spontaneous emission over a bandwidth around some center frequency and for guiding the light beam between opposed ends thereof; gain control means operably connected to the active section semiconductor means for supplying a current to the active section semiconductor means which will cause the active section semiconductor means to provide gain to the lightbeam sufficient to overcome cavity losses to provide lasing and an emitted beam of laser light; a pair of mirrors bounding the active section semiconductor means on respective ones of the opposed ends thereof, the mirrors each having narrow, spaced reflective maxima with the spacing of the reflective maxima of respective ones of the mirrors being different whereby only one the reflective maxima of each of the mirrors can be in correspondence and thereby provide a low loss window at any time, the mirrors each including a plurality of discontinuities to cause the reflective maxima, the end spacing of the discontinuities of one mirror being different from that of the other mirror so as to cause the spacing of the reflective maxima to be different in respective ones of the mirrors; vernier control circuit means operably connected to the mirrors for providing a voltage signal to the mirrors which will cause continuous tuning of the low loss window within a desired frequency band; offset control circuit means operably connected to the mirrors for providing a voltage signal to the mirrors which will shift the reflective maxima of the mirrors into alignment at a desired frequency mode; phase shifter means for adjusting the round trip cavity phase and thus the lasing mode wavelength; and, phase control circuit means operated in synchronism with said vernier control circuit means for aligning the wavelength of the lasing mode to be the same as the low loss window.
Diode lasers are being used increasingly in such applications as optical communications and sensors. In such applications, a serious problem exists with respect to the lack of a diode laser which can be operated at selectably variable frequencies covering a wide wavelength range. Without such a laser, the number of independent "channels" that can occupy a given wavelength range is exceedingly small. Accordingly, the number of individual communications paths that can exist simultaneously is, likewise, much smaller than possible. Thus, while diode lasers provide solutions to many problems that have plagued communications, sensor, and computer system designs, they have not fulfilled early promises based on the available bandwidth. In order for optical communication to become viable for many future applications, such as local area networks, the number of channels must be increased.
For a variety of applications, it is necessary to have tunable single-frequency diode lasers which can select any of a wide range of wavelengths. Such applications include sources and local oscillators in coherent lightwave communications systems, sources for other multi-channel lightwave communication systems, and sources for use in frequency modulated sensor systems. Continuous tunability is usually needed over some range of wavelengths; but, for some applications, a combination of discrete wavelength jumps combined with a more limited range of continuous tunability is acceptable. The continuous tuning is important for wavelength locking or stabilization with respect to some other reference, and it is desirable in certain frequency shift keying modulation schemes. If continuous tuning over the entire range of interest were possible, it clearly would be preferred, however.
A typical diode laser is shown in FIG. 1 where it is generally indicated as 10. The laser 10 has a mirror 12 at each end. A light beam 14 is created and amplified within the laser 10 by the pumping current source 16 operably connected thereto. The light beam 14 is created by a process termed "spontaneous emission" over a bandwidth around some center frequency. Some of this light is captured by the dielectric waveguide formed by the active medium. The guided light is reflected by mirrors 12 that create a Fabry-Perot resonant cavity. The modes of this resonant cavity are spaced nearly equally in frequency according to .DELTA.f=c/(2n.sub.g 1), where n.sub.g is the group index of the waveguide and 1 is the mirror spacing. Simply put, these mode frequencies are those for which 1 is an integer number of half wavelengths, or those at which the lightwave 14 adds constructively to itself after traversing the roundtrip of the cavity. Likewise, the mirrors 12 do not reflect 100% of the light striking them. As a consequence, there is some loss through the mirrors 12 at each reflection by the light beam 14. Also, there is propagation loss in the waveguide between the mirrors 12. As energy is added to the system by the current source 16, those frequency components of the light beam 14 which are in phase with the spacing of the mirrors 12 (i.e. Fabry-Perot modes) tend to be additive while those which are out of phase have components which tend to cancel out. The additive components continue to build in power until the laser "lases". That occurs at the current where the gain of the active medium equals the losses of the cavity waveguide and mirrors. Above this "threshold" current, the output laser light 18 from laser 10 increases rapidly in value.
FIGS. 4(a)-4(c) plot the mirror reflectivity, Fabry-Perot mode locations, and gain as a function of frequency for laser 10. FIG. 4(a) represents the reflectivity of the mirrors 12; that is, there is no substantial change in reflectivity over the gain bandwidth of the active medium. If one were to map or graph the corresponding loss of the laser 10, it would, of course, be the inverse of the graph of FIG. 4(a), having a flat loss. The Fabry-Perot modes of the laser light 18 can be depicted as shown in FIG. 4(b) for convenience, if desired. Without gain, all modes are equal. The overall gain of the laser 10 can be depicted as shown in simplified form in FIG. 4(c). At the maximum gain frequency, the gain minus loss reaches zero first as the gain level is increased by the current source 16, yielding an output 18 that contains a dominant mode near this frequency; however, the adjacent modes also have substantial gain. Thus, this is not a very good single frequency laser. The primary frequency of the laser 10 can be "tuned" only by changing the net optical cavity length as, for example, by adjusting the temperature thereof. This is certainly not an answer to the problem of providing a rapidly-tunable, single-frequency laser having selectable frequencies over a broad range.
As shown in FIG. 2, one can make a two part diode laser 28 by replacing one of the mirrors 12 with, for example, a grating 30, as depicted in FIG. 10. The overall reflectivity of such a mirror is shown in FIG. 4(d). The grating 30 provides a multiple reflective surface at the one end such that there are multiple spacings between the single mirror 12 and the multiple reflective surfaces of the grating 30. As can be seen from the figure, therefore, there is only one primary frequency band in the emitted laser light 18. This "Distributed Bragg-Reflector" (DBR) laser is good for single frequency emission; and, as is known to those skilled in the art, the two part diode laser 28 of FIG. 2 can be tuned to a small degree by electrically modifying the characteristics of the grating 30 (or other similarly operating device). A larger tuning is possible by using "jumps" to alternate modes. If, instead of using a grating the second mirror is composed of two discrete reflectors or an etalon, we get a multiple pass band mirror as shown in FIG. 4(e). The effective mirror on the right as the figure is viewed is then termed a "Fabry-Perot mirror". If the material with the Fabry-Perot mirror can be pumped to have gain, the configuration is generally referred to as a coupled-cavity laser. Tuning over several mode spacings is possible by tuning the currents to one or the other cavity by jumping from one mode to the other as with the grating two-section laser.
A substantial advancement in the art with respect to laser tunability was provided by the inventor herein by virtue of a three part laser as reported in an article in the IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, Jun. 1987 entitled "Continuously-Tunable Single-Frequency Semiconductor Lasers" by Larry A. Coldren and Scott W. Corzine (copy filed herewith). In that article, the present inventor, Dr. Coldren, and Mr. Corzine proposed a three section geometry, as depicted in FIG. 3, to provide "ideal" continuous tunability over a relative range of wavelengths .DELTA..lambda./.lambda..about.0.5.DELTA.n/n, where .DELTA.n is the maximum net index of refraction change that can be obtained in the passive sections of the laser, i.e. 32 and 34, respectively. The factor of .about.0.5 in the above .DELTA..lambda./.lambda. limit comes from the fact that the central phase shifter 32 must be modulated by about twice as much as the grating section 34 (which can be either a DBR grating reflector (FIG. 10) or a Fabry-Perot reflector (FIG. 11)) in order to line up a particular mode with the grating pass band since there is no tuning in the gain section 36. The gain section 36, phase shifter section 32, and grating section 34 are, of course, each "driven" by appropriate inputs at the inputs labelled "1", "2", and "3", respectively. If mode jumps are permitted, one can hope to gain back this factor of two. As is known in the art, the main limitation on tuning range is due to the limited net index change that can be obtained for a waveguide phase shifter. In forward biased carrier injection schemes, workers at NEC have been able to obtain an effective .DELTA.n/n.about.0.5%; but, free carrier loss is a major problem. In special reverse biased structures, the inventor herein and his workers have been able to obtain record levels of phase shift at low voltages .about.100.degree./Vmm, with a maximum effective .DELTA.n/n.about.0.05% at 5V; however, recent calculations by the same group indicate that relative index changes .about.5% might be possible in very exotic quantum-wire structures. Index changes .about.1% should be possible in less exotic quantum-well structures without undue loss. About half of these numbers might be available in a practical waveguided geometry. Thus, one tack that one might take in achieving a practical tunable laser adaptable for commercial applications would be to develop or find a very efficient phase shifter that might be integrated into a laser such as the prior art three section laser of FIG. 3.
With the more modest levels of phase shift available today, it was determined that it might be possible to cover a much wider wavelength range if discrete mode jumping could be combined in a practical way with continuous tuning. Many systems need a source that can emit wavelengths over a very wide range; but, it is not essential that the device tune smoothly from one point to another. It simply must get to the desired wavelength in a relatively short time. For packet switching kinds of applications, switching time .about.1 ms may be quite tolerable; however, it is important that there be no ambiguity between the control signals to the device and the resulting wavelength.
Some time ago, it was felt in the art that any wavelength over the gain bandwidth of a laser could be selected by a properly designed two-section, coupled-cavity structure using the combined mode-jump/continuous-tuning philosophy. We now know that it is not possible to get sufficient spurious-mode suppression and unambiguously select a particular wavelength over the entire band. The three section device of FIG. 3 with a Fabry-Perot (FP) mode selection filter (FIG. 11) as the "grating" 30 is a slightly better candidate for combining mode-jump tuning with continuous tuning for broader wavelength coverage since it can continuously tune over several longitudinal mode spacings. As in the simple two section case, however, the mode selection filter has multiple reflection maxima as depicted in FIG. 4(e). Thus, it is theoretically conceivable that one maximum could be used for one wavelength tuning band and another maximum could be used for another wavelength tuning band, etc. As a practical matter, however, it is apparent that some provision must be made for unambiguously deciding which filter reflection maximum is to be used. Also, some provision must be made for improving the suppression of adjacent modes in a practical, positive, and efficient manner. There are also problems of theoretical operation versus actual operation that must be addressed in bringing a tunable laser to a practical commercial level. For example, one can talk about continuous tuning within one band and then jumping to the next adjacent band. In actual practice, however, operation tends to "stick" at a particular mode's primary frequency and then suddenly jump to the next mode's primary frequency rather than tuning smoothly within the band. Likewise, there is a practical limit on how much tuning can take place with prior art lasers such as the three section laser geometry of FIG. 3. As will be recalled, the relative change in mode wavelength which can be effected by the phase shifter 32 is limited to approximately one-half of the change which takes place in it. This is a physical limit which is currently .about.1%.
Wherefore, it is an object of the present invention to provide a tunable semiconductor laser which has a broad wavelength bandwidth in which it operates.
It is another object of the present invention to provide a tunable semiconductor laser which operates in a manner which makes it commercially practical.
It is still another object of the present invention to provide a tunable semiconductor laser which operates in a combined mode-jump/continuous-tuning manner and which smoothly tunes continuously within each band before making a jump to the next band.
It is yet a further object of the present invention to provide a control system for a tunable semiconductor laser operating in a combined mode-jump/continuous-tuning manner which simply, quickly, and accurately causes the laser to jump from band to band.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.